Galaxies

Galaxies are massive astronomical systems of stars, gas, dust, & dark matter held together due to gravity. Galaxies come in a variety of shapes: spiral, elliptical, and irregular. They are important when it comes to the architecture and evolution of the universe.

1.0Understanding the Galaxies 

A galaxy is a huge gravitationally bound system of stars, gas, dust, and dark matter. It can have as few as a few million or as many as trillions of stars. It is the major structure for cosmic matter and energy. The stars orbit around a common centre (Most of them contain supermassive black holes), and the galaxy may have various structures depending on its type.

2.0Galaxy Size And Composition

The sizes of galaxies can vary between a few thousand and hundreds of thousands of light-years in diameter. The galaxy is composed of the cosmic material of the universe, such as: 

• Stars: The most basic constituents of a galaxy. They vary in size, temperature, and age.
• Gas and Dust: These are essential for the formation of stars. They are present in the interstellar medium, filling the space between stars.
• Dark matter: Dark matter is a mysterious and invisible substance; hence cannot be observed by traditional telescopes but is thought to constitute most of the mass of a galaxy, affecting its gravitational pull and structure. The origin of Dark matter is not clear, but there are some theories that suggest that it was formed during the Big Bang. Although it can not be seen, some phenomena, like the rotation of speeds of galaxies, are much faster than expected, and gravitational effects on visible matter, such as galaxies and galaxy clusters, are not justified if only the visible matter is taken into account.  

3.0Galaxy Formation And Evolution

• Formation: Galaxies emerged from small fluctuations in the density of matter immediately after the Big Bang, roughly 13.8 billion years ago. As matter began to clump together by gravity, stars, gas, and dust formed galaxies.
• Mergers: The merger of galaxies is the process of collision and combination of two or more galaxies to form larger galaxies due to the effect of gravity. During these mergers, the cloud of dust and gas experiences large amounts of interactions and compression that can initiate the process of star formation and make the structures more complex. The mergers are very common but take a long time scale like billions of years. 

• Evolution: Galaxies change with time through star birth, supernovae, and mergers with other galaxies.

4.0Colliding Galaxies

As mentioned above, galaxies can merge to form larger galaxies, a process that can trigger star formation and create more complex structures. This is an important process in the evolution of galaxies and may eventually lead to the formation of elliptical galaxies from spiral galaxies.

For example, The Milky Way Galaxy and the Andromeda Galaxy are on a collision course, set to collide in about 4 billion years.

5.0Types of Galaxies

Among the different Galaxies types, we have enlisted some major and important ones below.

1. Spiral Galaxies: These galaxies contain a flat rotating disk of spiral arms. The Milky Way is one example of a spiral galaxy. Often, the centre will be a bulge of older stars.

Example: Milky Way, Andromeda Galaxy.

2. Elliptical Galaxies: These galaxies have round or oval shapes and an evenly spread distribution of stars. They are known to harbour older stars with almost no interstellar gas and dust.

Example: M87.

3. Irregular Galaxies: They are not of well-defined shape and are even messy. They have usually high rates of star formation.

Example: Large Magellanic Cloud.

4. Lenticular Galaxies: These are galaxies intermediate between the spiral and elliptical galaxies. These contain a central bulge with a disk but they lack prominent spiral arms.

Example: NGC 1023.

6.0Galaxies In The Universe 

There are billions of different galaxies in the observable universe and trillions of galaxies that still need to be discovered each containing billions of stars. Some of the most famous galaxy names are: 

• Milky Way – our Home 
• Andromeda Galaxy (M31)
• Triangulum Galaxy (M33) 
• Sombrero Galaxy (M104) 
• Whirlpool Galaxy (M51) 

7.0Largest Galaxies

• The largest galaxies are supergalaxies and giant elliptical galaxies.
• IC 1101 is one of the largest known galaxies, measuring over 6 million light-years in diameter.

• Coma Cluster's Elliptical Galaxies: The Coma Cluster hosts some of the largest elliptical galaxies known, such as NGC 4874 and NGC 4889.